Flat panel displays are an enabling technology for all contemporary portable consumer electronic devices and large-format televisions. Silicon crystallization is a processing step that is often used in the manufacture of thin-film transistor (TFT) active-matrix liquid-crystal displays (AMLCDs) and active-matrix organic light-emitting diode (AMOLED) displays. Crystalline silicon forms a semiconductor base, in which electronic circuits of the display are formed by conventional lithographic processes.
Commonly, crystallization is performed using a pulsed beam of laser-radiation that is shaped into the form of a long line having a uniform intensity profile along the length direction (long-axis) and a uniform or “top-hat” intensity profile across the width direction (short-axis). In the crystallization process, a thin layer of amorphous silicon (a “silicon film”) on a glass substrate is repeatedly melted by the pulsed laser-radiation, while the substrate and the silicon layer thereon are scanned relative to a source and optics delivering the pulsed laser-radiation. Repeated melting and re-solidification (recrystallization) through exposure to the pulsed laser-radiation, at a certain optimum energy-density, take place until a desired crystalline microstructure is obtained in the silicon film.
Optical elements are used to form the pulsed beam of laser-radiation into a long line on the silicon film. Crystallization occurs in a strip having the length and width of the long line of laser-radiation. Every effort is made to keep the intensity of the pulsed laser-radiation highly uniform along the long line. This effort is necessary to keep the crystalline microstructure uniform. A favored source of the pulsed laser-radiation is an excimer laser, which delivers laser-radiation having a wavelength in the ultraviolet region of the electromagnetic spectrum. The above described crystallization process, using excimer-laser pulses, is usually referred to as excimer-laser annealing (ELA).
The process is a delicate one. The error margin for the optimum energy-density can be a few percent or even as small as ±0.5%.
In a typical example of ELA, the “line-length” (long-axis dimension) of the beam is in the range of about 750 millimeters (mm) to 1500 mm. The “line-width” (short-axis dimension) of the beam is about 0.4 mm. The pulsed laser-radiation has a duration of about 50 ns and a pulse repetition frequency of about 500 Hertz (Hz), i.e., the pulses are temporally separated by about 2 milliseconds (ms). A substrate and a silicon layer thereon are scanned perpendicular to the long-axis of the beam at a rate such that any location on the silicon layer is irradiated by about 20 consecutive pulses, thereby recrystallizing the silicon layer. The process is illustrated schematically by FIG. 1A and FIG. 1B.
FIG. 1A schematically illustrates a typical short-axis intensity profile 12 of an above-discussed excimer laser beam. The beam is characterized by a line width W, here, measured between half-maximum intensity points of the intensity profile. The scan direction of a workpiece (a substrate with a silicon layer thereon) being irradiated is indicated by arrow A. FIG. 1B schematically illustrates the above discussed pulse overlapping scheme with any location on the substrate being irradiated by the about 20 consecutive pulses. Methods and apparatus for shaping excimer laser beams are described in U.S. Pat. Nos. 7,615,722 and 7,428,039, assigned to the assignee of the present invention, and the complete disclosures of which are hereby incorporated by reference.
Methods and apparatus for monitoring and controlling output of an excimer laser to about the above-discussed ±0.5% tolerance have been developed and are in use. Nevertheless, there remains a continuing need to improve such methods and apparatus for producing large area crystalline silicon layers with improved manufacturing yield.